1. Field of the Invention
This invention relates to a process for polymerizing ethylene at a temperature not lower than 125.degree. C. and under a pressure not lower than 200 kg/cm.sup.2 to produce polyethylene having a high molecular weight.
2. Prior Art
Recently, a process for polymerizing ethylene at a high temperature and under a high pressure in the presence of a Ziegler-type catalyst by using a conventional high-pressure ethylene-polymerization apparatus, as described in British Pat. No. 828,828 Specification, etc. has been proposed.
This method is advantageous for industrial production of linear-chain low-density polyethylene (LLDFE) for the following reasons.
First of all, conventional high-pressure polyethylene-producing apparatus can be used as they are; no investment in equipment is needed. Secondly, although removal of heat is a serious problem in the polymerization process because of the exothermic reaction of ethylene polymerization, by this method polymerization can be conducted at a high temperature to increase the temperature difference between the inner temperature and that of a cooling carrier, which results in increasing efficiency in removal of heat and, thus, enhancing conversion to polymers. Thirdly, it is not necessary to remove a solvent from the polymer produced, and the polymer can be readily separated. Fourth, because the resulting polymer is in a molten state, it is not necessary to melt the polymer for pelletizing unlike in the cases of liquid phase polymerization and suspension polymerization, and, thus, the method is advantageous with respect to energy.
On the other hand, a problem in high-temperature and high-pressure polymerization process is that polymers having a sufficiently low melt-flow index (hereinafter, abbreviated to MFR) cannot be produced since the chain transfer rate of ethylene is much greater than the growth rate of ethylene at a high temperature. This problem is particularly serious in the copolymerization of ethylene with an .alpha.-olefin. This is because the chain transfer rate of .alpha.-olefins is greater than that of ethylene, and, thus, lowering of the MFR of the resulting polymer becomes more difficult. The problem of the MFR of the resulting polymer being too high can be solved with a low polymerization temperature. Such solution of the problem, however, sets the upper limit of the polymerization temperature with respect to the MFR of the desired polymer, resulting in decrease in conversion to polymer.
Another problem in the above mentioned polymerization under high-temperature and high-pressure is that it is difficult to omit the step of removing catalyst residues. In the case where the catalyst employed has a low polymerization activity, a large amount of the catalyst residue remains in the resulting olefin polymer which raises serious problems with respect to the properties of the polymer product such as thermal stability, color, odor and the like.
Moreover, high-temperature and high-pressure polymerization is apt to produce a waxy polymer having a low molecular weight. Formation of such a low molecular-weight waxy polymer has an adverse effect on the commercial quality of the resulting polymer product.
In addition, there is a further problem in that the high-temperature and high-pressure polymerization produces a large amount of double bonds in the resulting polymer. When a large amount of double bonds is present in the polymer, especially inside of the polymer chain, the polymer is apt to oxidatively decompose and have reduced weather resistance.
Thus, it is important for industrialization of the above mentioned art to develop a catalyst which, in high-temperature high-pressure polymerization, has a high activity as well the capability to produce polymers having a broad range of MFR including a low MFR region and excellent quality.